Many electronic systems, such as servers or microservers, require a reliable and uninterrupted source of power. In a power supply system providing uninterrupted power supply, backup or redundant power supplies are mounted in parallel with the main power bus line. FIG. 1 illustrates a conventional power supply system where a redundant power supply is coupled in parallel to the main power supply line. Referring to FIG. 1, a power supply system 1 includes a main power supply line 2 and a redundant power supply 3. The redundant power supply 3 is usually mounted in parallel using an OR'ing method, denoted by OR'ing circuit 4. The backup power supply 3 remains dormant and on standby mode for the most part of its existence and is only in operation when the main line 2 experiences an unscheduled downtime or when maintenance services are performed. The purpose of the redundant supply 3 is to immediately turn on in the event that the main power line 2 experiences fault, hence, providing uninterrupted operation or service to the end user. Typically, the redundancy power line 3 has the ability to flag or notify the system that it has been enabled, thus allowing for maintenance or service to the main line 2 at a more convenient time.
Mounting the redundant power supply in parallel to the main line by means of OR'ing can be done by the conventional diode method, which provides a blocking mechanism when there are no fault conditions. However, during a fault condition, the drop across the diode can result in significant power loss, especially when the power system is operating at a high current level, such as the current level often required by micro-servers and other hardware in telecommunication applications, which can be well above 80 A.
The OR'ing of the redundant power supply can also be done by using a MOSFET switch with very low ON-state resistance. Using a MOSFET switch for the OR circuit will require good control and sense circuitry that is governed by accurate timing and sequencing. The MOSFET OR'ing scheme provides lower power loss and improves efficiency during “backup” events but can be disadvantageous during the unlikely event that the secondary (redundant) line's input voltage fails and drops, causing reversal of current since the MOSFET switch allows for bi-directional conduction. MOSFET switch in series with back to back body diodes can be an option to prevent reverse current flow from occurring but will require timing circuitry for accurate sequencing.
The redundant power supply and the OR'ing circuit increases system cost in addition to consuming real estate space, the cost and space being used for a redundant system that is primarily inactive. Data centers and telecommunication infrastructure applications strive for lower server profile, tighter rack space, higher power density designs and lower cost. The requirement to include a redundant line increases the complexity of the power supply system as an intelligent controller for the fault detect circuits, current sensing capability for current limiting, current reversal detectors, break before make housekeeping circuits for accurate timing and other aforementioned circuits accessories need to be incorporated in the redundant power supply system. The complexity of the redundant power supply increases system cost and takes up valuable PC board real estate.